Why did the Greenlanders leave Greenland?

An object lesson for California, it was due to this:

In the final phase, it was young people of child-bearing age in particular who saw no future for themselves on the island. The excavators found hardly any skeletons of young women on a cemetery from the late period.

"The situation was presumably similar to the way it is today, when young Greeks and Spaniards are leaving their countries to seek greener pastures in areas that are more promising economically," Lynnerup says. "It's always the young and the strong who go, leaving the old behind."